In a case of evaluating objects by using a spectrometer, light in a near-infrared region is less absorbed by moisture. Therefore, a near-infrared spectrometer using a near-infrared luminescent device is particularly suitable for evaluating objects, which contain a large amount of water, such as biological objects, fruits and vegetables.
In addition, since an optical coherence tomography (OCT) uses near-infrared light, the optical coherence tomography (OCT) has characteristics that biological damage thereof is lower than that of a general X-ray tomography, and spatial resolution of tomographic images is excellent.
Further, in a solar battery or a solar power generator using silicon, photo-responsibility of silicon in a near-infrared region is the highest. Therefore, if visible light can be converted into near-infrared light, the power generation efficiency can be further enhanced.
Meanwhile, when a phosphor capable of emitting near-infrared light is used, a novel fluorescent paint or fluorescent paint printed matter can be developed. For example, conventionally, a UV light fluorescent pigment has been mainly used for forgery-proof printing utilized for paper-money or the like. If this can replace with near-infrared fluorescent pigment, novel forgery-proof printing in which the forgery, which cannot be detected with the naked eye, can be detected by a solid-state imaging device using a photo-diode made of silicon, InGaAs or the like, or equipment using a photomultiplier tube, becomes possible.
As such, it is expected that the near-infrared luminescent phosphor, which is excited by visible light, capable of emitting near-infrared light can be used for various applications in the future.
For example, Japanese Patent Application Laid-Open No. 2008-185378 discloses a near-infrared luminescent phosphor which contains Yb2O3 and Nd2O3as an infrared glass phosphor used for OCT apparatus, further contains glass comprising Bi2O3 and B2O3, and is excited by blue light. Further, Japanese Translation of PCT International Application Publication No. 2004-526330 discloses a near-infrared luminescent body which is near-infrared light-excited by a glass-ceramic material in which transition metal ions are doped, and also proposes its expansion to OCT apparatus.
These phosphors have an advantage of having a wide fluorescence spectrum. However, when the phosphors are practically used, there has been a problem that the excited spectrum is narrow. Particularly, there has been a demand of developing a phosphor that is excited by a wide band of visible light, and has a wide fluorescence spectrum.
However, there are few kinds of near-infrared luminescent phosphor which have been conventionally known. For example, in the aforementioned Japanese Patent Application Laid-Open No. 2008-185378, the infrared glass phosphor which contains Yb2O3 and Nd2O3 as the infrared glass phosphor used for OCT apparatus, and further contains glass comprising Bi2O3 and B2O3, has been disclosed at best.
Thus, the present invention provides a novel phosphor, which is excited by visible light of a wide band, capable of emitting near-infrared light with high intensity.